The invention relates to an arrangement for coupling a plurality of transmitters having different carrier frequencies to an aerial according to the preamble of claims 1 and 2.
In future radio transmission systems having digital or analog speech transmission on the radio transmission path, for example in car phone systems, it must be possible to radiate a very great number of ratio transmission channels from the stationary radio station. In order to radiate as many radio transmission channels as possible from a single aerial, a plurality of high-frequency ratio transmission channels are combined in a transmitter coupler. In DE-PS 28 44 776, corresponding to U.S. Pat. No. 4,211,894, a transmitter coupler is disclosed, comprising a plurality of branches each having a circulator network and a band-pass filter. Multiplexing the output signals of the various transmitters having different carrier frequencies is carried out by the band-pass filters, which have one end connected to a star network and from there to the aerial via an aerial transmission line, and the other end to the transmitters via circulators.
In order to achieve mutual decoupling of the radio transmission channels the individual channel frequencies at a transmitter coupler must be spaced by a multiple of a channel spacing of the system (coupler channel spacing). The maximum number of radio transmission channels connected to a transmitter coupler depends on the permissible insertion loss and the available bandwidth of the radio transmission system. In practice, a power loss of approximately 3 dB can be found when multiplexing. This power loss cons of the loss in the Y circulator of 0.5 dB, the loss in the band-pass filter of approximately 1.5 dB and other losses such as reflexion losses of approximately 1 dB.
Only such a number of radio transmission channels can be simultaneously connected to a transmitter coupler as is the result of the division of the bandwidth of the radio transmission system by the coupler channel spacing. If the coupler channel spacing is reduced the effect of the transmitter coupler is increased, but on the other hand, if there is a greater number of channels the insertion loss of the transmitter coupler will be greater too, increasing the power loss. Reduced insertion losses can be achieved by using band-pass filters having larger dimensions, making the transmitter coupler more expensive and also requiring more space.
The band-pass filters are adjusted to a specific operating frequency and can only work at another operating frequency after they have been adjusted anew. If an increased system capacity can be achieved by dynamic channel assignment, further transmitter coupler inputs can be provided, which are then utilized only occasionally. When this measure is implemented, more space will be required, stiffer demands will be made on the coupler channel spacing and the cost of the transmitter coupler will go up, so that a dynamic channel assignment in radio transmission systems has not been utilized so far.
In DE PS 28 44 776 a hybrid circuit is connected behind the transmitter coupler for switching operations between the various frequency groups, this hybrid circuit operating as a power divider, more specifically, a branching circuit, thus achieving a spreading of the frequency groups over the respective directional aerials.
As the transmitter coupler operates channel selectively in the output frequency range, each individual radio transmission channel has to be amplified accordingly prior to the transmitter coupler. The subject matter of DE PS 28 44 776 does not provide a multiple use of the power amplifiers and integration of the transmitter coupler is not possible either. By using a suitable dielectric, though, the outside dimensions of, for example, an air-filled band-pass filter can be reduced.
From DE-PS 26 39 348, corresponding to U.S. Pat. No. 4,148,038, a circuit arrangement is known for connecting a plurality of mutually decoupled transistors to a single aerial system utilizing four 3 dB couplers. Hybrid circuits are used as 3 dB couplers, that is to say branching circuits in the .pi./4 stripline technique, to which the four individual transmitters and the four individual aerials are connected in a specific way. The hybrid circuits have three lines .pi./4 in length as well as one line three .pi./4 in length. The hybrid circuits equally divide between their two outputs the power received from the transmitters connected to them, and the phase shift in the ring causes the fading waves to be cancelled. In a circuit arrangement known from DE OS 30 04 817, the course indicated by the subject matter of DE PS 26 39 348 is continued and a transmitter coupler in a cascade configuration with phase-shifting circuit components is proposed.
As examinations of the circuit arrangements known from DE-PS 26 39 348 and DE-PS 30 04 817 have shown, they have a high reflexion factor, caused by reflexion to the hybrid circuits or directional couplers, respectively, connected to form the ring or to the aerials. A further disadvantage is the fact that within the reception area there may be zones in which the transmission signal radiated by the aerial is completely cancelled. This can be attributed to the fact that no omnidirectional characteristic can be achieved with the aerial configuration of four directional aerials, in which such cancellations due to reflexion to buildings, trees etc. may occur as a result of the multipath reception. The power loss of such transmitter couplers composed of hybrid circuits or directional couplers, respectively, is due to the order of magnitude of conventional transmitter couplers with a circulatory network and band-pass filter.
The energy radiated by convection in case of a great number of channels and a high packing density has to be eliminated by means of forced cooling. As the band-pass filters are adjusted to the output frequency band, it is impossible to re-arrange channels, for example, for avoiding common-channel interference, so that a manual re-adjustment is always required for a different output frequency.